The pituicyte of the pituitary gland's neural lobe, resembles the more common astrocyte of the CNS in being perivascular and in having intramembranous, orthogonal aggregates of particles or assemblies. However, pituicytes subtend fenestrated vessels permeable to peptides and have dynorphin receptors and cells processes that embrace the neurosecretory terminals (NST) of the neural lobe. It is known that in a dehydrated rat, these processes retract, exposing the NST directly to the fenestrated capillaries. During rehydration, the processes reensheath the NST. We have examined explants to determine the mechanism of reversible contraction and to detect whether Ca + + is involved. We have found that, like other astrocytes, pituicytes change shape from epithelioid to stellate in serum-deprived culture medium. If the pituicytes are initially kept in 5% fetal calf serum instead of the usual 10%, the shape change is completed within one hour. By the use of video-enhanced differential interference contrast and time-lapse microscopy, we have been able to follow the conversion of lamellipodia to thin, varicose branches. A less pronounced change is caused by dibutyryl cAMP in the presence of serum. If Ca + + is involved in process retraction, we may then see whether there are Ca + + channels sensitive to dynorphin and vasopressin, that are co-released during neurosecretion, and to oxytocin and catecholamines by the use of calcium activated flurochromes (e.g., Fura-2) in a video-enhanced microscope system.